streams, rare on recently inundated soil and on moist earth from 

 which the water has receded. The colonies often appear as marble- 

 like bodies scattered over the bottom among submerged, grass, 

 reeds, etc., sometimes in large numbers so that one can scoop them 

 in double handfuls. Although the average size is 1-2.5 cm., the 

 colonies may attain a diameter of 5 cm. Not infrequently they are 

 mistaken for reptile eggs. 

 Mich., Wis. 



Nostoc punctiforme ( Kuetz. ) Hariot 1891, p. 31 

 PI. 121, Figs. 4, 5 



A small, usually attached, colony of very compactly entangled 

 trichomes in a colorless, soft mucilage; the linear arrangement of 

 the cells frequently not discernible. Cells compressed-globose, 3.2-4/* 

 in diameter. Heterocysts globose, 4.5-6/t in diameter. Gonidia ovate 

 or oblong; 5-6/a in diameter, 5-8/* long. 



Attached to large filamentous algae and to leaves and stems of 

 submerged aquatics in hard water lakes. This species also occurs 

 as an endophyte or a symbiont with lichens and some cycads. Mich., 

 Wis. 



Nostoc sphaericum Vaucher 1803, p. 223 

 PL 121, Figs. 6-9 



A globose or tubercular, olive-green colony when young, becoming 

 flattened and somewhat membranous and brown in age; trichomes 

 densely entangled; cells globose, 4-5/* in diameter; heterocysts 

 spherical, 7.4/* in diameter; gonidia solitary or in very short series 

 (3-4 together), ovate, 5-7/x in diameter, 10/* long, with a smooth 

 brown wall. 



The colonies become as much as 5-6 cm. wide under favorable 

 conditions. The species should be compared with N. pruniforme, an 

 aquatic species, but one which sometimes appears in the same habi- 

 tats as N. sphaericum. 



Growing on soil at the margins of lakes; in marshy places among 

 grasses; in thalli of Ricciocarpus. Mich., Wis. 



Nostoc spongiaeforme C. A. Agardh 1824, p. 22 

 PI. 121, Fig. 10 

 A globular colony of loosely entangled trichomes when young, 

 becoming lobed, expanded, warty and bullate, especially when grow- 

 ing on damp soil, the color changing from blue-green to brownish- 

 green. Cells variable in shape within the same colony, subglobose to 

 barrel-shaped and cyhndrical; 3.4-5/* in diameter, 5.4-6.5/* long. 

 Heterocysts ovate to oblong or subglobose, 4.8-8/* in diameter, 7.2/* 



[525] 



